Applications of Polarization FFF

The character of the applied field determines the particular methods of polarization FFF. The most important of them are:

• sedimentation FFF

Sedimentation FFF is based on the action of gravitational or centrifugal forces on the suspended particles. The sedimentation velocity is proportional to the product of the effective volume and density difference between the suspended particles and the carrier liquid. The channel is placed inside a centrifuge rotor, as shown in Figure 4. The technique can be used for the separation, analysis and characteriza-

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Figure 3 Dependence of the efficiency of FFF, expressed as the height equivalent to a theoretical plate H, on the average linear velocity of the carrier liquid <v(x)>.

tion of polymer latex particles, inorganic particles, emulsions, etc. The fractionation of colloidal particles in river water, diesel exhaust soot, and of the nuclear energy-related materials, are typical examples of the use of sedimentation FFF in the investigation of environmental samples. Droplets of liquid emulsions can also be separated and analysed. Biopolymers and particles of biological origin (cells) belong to the most interesting group of objects to be separated by sedimentation FFF. The performance of sedimentation FFF is superior to, or as good as, those of other separation methods. A complication in interpreting the experimental data is due to the fact that the retention is proportional to the product of particle size and density. When performing the fractionation in one carrier liquid only, the density must be assumed constant for all particles. However, it is possible to determine the size and density of the particles independently if the fractionations are performed in carrier liquids of various densities.

An example of a typical application of sedimentation FFF shown in Figure 5 allowed detection of a bimodal PSD in a sample of a polymer latex. The order of the elution from the small to the large diameter particles corresponds to the polarization mechanism. Figure 6 shows a rapid, high resolution sedimentation FFF of the polymer latex particles. In this case, the mechanism of steric FFF dominates, and the order of the elution is inverted.

Flow FFF is a universal method because different size particles exhibit differences in diffusion coefficients which determine the separation. The cross-flow, perpendicular to the flow of the carrier liquid along the channel, creates an external hy-drodynamic field which acts on all particles uniformly. The channel, schematically demonstrated in Figure 7, is formed between two parallel semipermeable

Figure 5 Fractogram of poly(glycidyl methacrylate) latex showing a bimodal character of the PSD.

Figure 5 Fractogram of poly(glycidyl methacrylate) latex showing a bimodal character of the PSD.

membranes fixed on porous supports. The carrier liquid can permeate through the membranes but the separated particles cannot. Separations of various kinds of particles such as proteins, biological cells, colloidal silica, polymer latexes, etc., have been described.

Electric FFF uses an electric potential drop across the channel to generate the flux of the charged particles. The walls of the channel are formed by semipermeable membranes as in flow FFF. The particles exhibiting only small difference in elec-trophoretic mobilities but PSD and, consequently, important differences in diffusion coefficients, can be determined. The advantage of electric FFF compared with electrophoretic separations, e.g., with capillary electrophoresis, is that high electric field strength can be achieved at low absolute values

of the electric potential due to the small distance between the walls of the channel. Electric FFF is especially suited to the separation of biological cells as well as to charged polymer latexes and other colloidal particles. The fractionation of the charged particles represents a vast application field for exploration.

Thermal FFF was the first experimentally implemented technique, introduced several years ago. Until now, it has been used mostly for the fractionation of macromolecules. Only very recently have attempts been made to apply this method to the fractionation of particles. The potential of thermal FFF justifies a description here, regardless of its recent limited use in particle separations. The temperature difference between two metallic bars, forming channel walls with highly polished surfaces and separated by a spacer in which the channel proper is cut, produces a flux in the sample components, known as the Soret effect, usually towards the cold wall. The particle sizes can be evaluated from an experimental fractogram by using an empirical calibration curve constructed with a series of samples of known sizes. This calibration can be used to determine the characteristics of an unknown sample of the same chemical composition and structure, with the same temperature gradient applied. The pressurized separation systems permit operation above the normal boiling point of the solvent used. The fractionations can be achieved in few minutes or seconds. The performance parameters favour thermal FFF over competitive methods.

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